Spring doesn’t always arrive on the same calendar date. You might remember it beginning on March 21, but more often lately it starts on March 20—or even March 19 in rare years. Why does that happen?
The shifting first day of spring arises from the mismatch between Earth’s true orbit and our calendar, time zones, leap years, and subtle gravitational nudges. In this article, you’ll learn exactly why the first day of spring changes, how the calendar handles that shift, and what impacts we see on holidays and nature in this rotation of seasons.
What Determines the First Day of Spring? Astronomical vs Meteorological
There are two main ways people define spring.
Astronomical spring is based on the vernal equinox—the moment when the Sun crosses the celestial equator from south to north. That marks equal day and night and signals the start of spring in the Northern Hemisphere.
Meteorological spring is simpler: it begins on March 1 and ends on May 31. Weather services prefer this fixed system because it aligns better with temperature patterns and makes climate data easier to compare year by year.
Most confusion comes from the astronomical spring, because that date shifts from year to year. Let’s explore why.
Earth’s Orbit Is Not 365 Days Exactly
A key reason the spring date changes is that Earth’s orbit around the Sun doesn’t take exactly 365 days—it takes about 365.2422 days (365 days, 5 hours, 49 minutes, and 9 seconds). Because of that fractional extra time, the equinox drifts later in the calendar each year by roughly 5 hours 49 minutes.
If we didn’t correct for that drift, eventually spring would occur much later in the calendar, pushing March into June over many centuries. To prevent that, the Gregorian calendar adds a leap day roughly every 4 years (February 29). That extra day resets the drift—but it doesn’t eliminate it entirely. Over a 4-year period, the equinox usually jumps back earlier by up to 18 hours in a leap year.
So each year, the equinox time shifts forward by about 5 hours 49 minutes; in leap years, it shifts backward more because of the extra day. That interplay causes the equinox—and therefore the first day of spring—to float between March 19, 20, and 21.
Time Zones and Universal Time Differences
Even though the equinox happens at one exact moment worldwide, local time zones push the calendar date up or down. The event may occur late at night UTC, but in U.S. time zones it might already be the next calendar day—or still the previous one.
For example, if the equinox occurs on March 20, 1 a.m. UTC, in some U.S. zones it might still be March 19, or in others already March 20. That is why you sometimes see spring begin on March 19, 20, or 21, depending on where you live.
Thus, the combination of orbital drift and time zone offset is what makes the first day of spring non-fixed.
Examples: How the Date Moves Year to Year
Let’s look at how these mechanics play out.
- In a common year (non-leap), the equinox time drifts about 5 hours 49 minutes later, pushing the date forward one slot every few years.
- In a leap year, the calendar adds an extra day in February, which often pushes the equinox back by around 18 hours relative to calendar time.
- Over a century, that drift plus corrections means March 21 becomes rarer. Between 1981 and 2102, spring begins on March 21 only in a few years—most years it will fall on March 20 or even March 19.
For example, in 2024 the equinox fell on March 19—the earliest in 128 years. In 2025, it arrives March 20 at 5:01 a.m. EDT.
Because of that, March 21 is becoming less common as the spring start date—yet many still assume spring begins then out of habit.
Why It Matters: Holidays, Calendars, and Culture
This variable date has implications beyond astronomy.
- Easter: In Western Christianity, Easter is calculated based on the first full moon after the vernal equinox (traditionally March 21). Because the equinox can shift, Easter’s date shifts too.
- Cultural holidays: Some traditions, like Nowruz (Persian New Year), tie to the precise equinox moment and thus adjust year to year.
- Agriculture and planting traditions: Farmers historically used the fixed March 21 marker to plan planting. But now the shifting date means cultivating calendars must adapt.
- Public perception: Many people still believe spring always starts March 21. That persisted for centuries while calendars were less precise. But now, with better astronomy, we see the shift.
Though meteorologists use March 1 for convenience, the astronomical definition drives how we see seasons change in reality.
Sunlight, Day Length, and the Earth’s Tilt
On the equinox, both hemispheres receive nearly equal sunlight because Earth’s tilt is perpendicular to the Sun. Day and night lengths are nearly equal (though atmospheric refraction and solar size make day slightly longer in practice).
After the equinox, in the Northern Hemisphere, daylight exceeds darkness until the summer solstice. That increasing daylight drives the warming that we perceive as spring.
Because Earth’s orbit is elliptical and gravitational influences from the moon and other planets slightly distort our orbit, the exact equinox times wobble. That adds subtle variation too.
Why We Rarely See March 21 Equinox Now
Historically, March 21 was a common equinox date. But as our calendar rules adjust for orbital drift, March 21 equinoxes become rarer. In the 20th century, March 21 happened 36 out of 100 years.
Between 1981 and 2102, spring in the U.S. will start no later than March 20, and in some years on March 19—making March 21 almost exceptional.
So if you grew up believing spring always began March 21, you’re witnessing a shift in calendar reality—the calendar is catching up with astronomical truth.
Summary: What Causes the Change?
Here’s a concise breakdown.
- Orbital year ≠ 365 days → about 365.2422 days
- Drift accumulates ~5 hours 49 minutes per year
- Leap year adjustment resets some drift
- Time zones shift local date relative to UTC
- Gravitational perturbations add minor variation
- Result: the vernal equinox—and thus the first day of spring—shifts among March 19, 20, or 21
Thus, the first day of spring changes because the astronomical event moves relative to our fixed calendar.
What You’ll See Differently Because of This Shift
- Spring fairs, festivals, and planting may drift slightly in timing each year
- Easter and fixed holidays tied to equinox must adjust
- Cultural traditions that used to assume March 21 may fall “off”
- Public confusion may persist—some still over-rely on March 21 as default
Yet the changing date doesn’t affect spring’s experience—warming weather, blossoming plants, and longer daylight remain the same. We just mark it more precisely now.
Conclusion
The first day of spring changes because Earth’s true orbit doesn’t align perfectly with our calendar. The fractional extra time, leap year corrections, time zones, and minute orbital wiggles all push the vernal equinox—and thus spring’s start—around March 19 to 21.
As our calendars evolve to match astronomy more closely, we’ll see March 21 equinoxes grow rare. But regardless of the date, spring’s arrival brings renewal, longer days, and a fresh season of life.